A conventional color image forming apparatus is disclosed, for example, in Publication of Unexamined Patent Application (Tokkai) No. Hei 7-36246.
FIG. 14 explains such a conventional color image forming apparatus. As shown in FIG. 14, this apparatus comprises an intermediate transfer belt unit 201 including an intermediate transfer belt 202, a primary transfer roller 203, a secondary transfer roller 204, a cleaner roller 205, and a waste toner reservoir 206. Superimposition of color images is performed on the transfer belt 202. In the middle of the printer, a group of image forming units 208 is provided. Four image forming units 207Bk, 207Y, 207M and 207C for black, yellow, magenta and cyan, each unit being of sector shape in cross section, are arranged circularly to form a group of image forming units 208. When an image forming unit 207Bk, 207Y, 207M or 207C is installed properly in the color image forming apparatus, mechanical driving systems and electrical connection systems are coupled between the image forming units 207Bk, 207Y, 207M and 207C and other portions of the color image forming apparatus via mutual coupling members, so that both sides are mechanically and electrically connected. The image forming units 207Bk, 207Y, 207M and 207C are supported by a supporter and collectively rotated by a motor, so that they can revolve around a non-rotatable cylindrical shaft 209. For image formation, the image forming units 207Bk, 207Y, 207M and 207C are successively moved by rotation to an image forming position 210, where they oppose the primary transfer roller 203 spanning the intermediate transfer belt 202. The image forming position 210 is also the exposure position for exposure with a laser beam 211.
A laser exposing device 212 is arranged horizontally below the group of image forming units 208. The laser signal beam 211 passes through a light path opening 213 between the magenta and cyan image forming units 207M and 207C, and through an opening provided in the cylindrical shaft 209, and enters a mirror 214, which is fixed inside the shaft 209. The laser signal beam 211 reflected by the mirror 214 enters the black image forming unit 207Bk located at the image forming position 210 through an exposure opening 215. Then, the laser signal beam 211 passes through a light path between a developing device 216 and a cleaner 217, arranged on the upper and the lower side in the image forming unit 207Bk, enters an exposure portion on the left side of a photoconductive drum 218, and scans for exposure along the direction of the axis of the photoconductive drum 218. The toner image formed on the photoconductive drum 218 is transferred to the intermediate transfer belt 202. Then, the group of image forming units 208 rotates 90 degrees, so that the yellow image forming unit 207Y moves into the image forming position 210. An operation similar to the above formation of the black image is performed to form a yellow toner image overlaying the black toner image previously formed on the intermediate transfer belt 202. Similar operations as explained above are performed using the magenta and cyan image forming units 207M and 207C to compose a full color image on the intermediate transfer belt 202. After the full color image on the intermediate transfer belt 202 is completed, a recording paper is conveyed by a secondary transfer roller 204 and a tertiary transfer roller 219, and the color image is simultaneously transferred onto the recording paper. The recording paper onto which the color image has been transferred is conveyed to a fuser 220, which fuses the color image on the recording paper.
In an example of a conventional mechanism for coupling and driving, FIG. 15 shows how the driving mechanism on the main body side is coupled to the photoconductive drum. In FIG. 15, the main shaft 235 of the photoconductive drum 218 that has arrived at the image forming position 210 (see FIG. 14) is positioned by some means not illustrated in the drawing, and a gear 232, which is fixed to one end of the main shaft, engages a gear 241 provided at an output shaft 245 on the main body side. Thus, a driving force is transmitted from the main body side to the photoconductive drum 218.
In Publication of Unexamined Japanese Patent Application No. Hei 2-12271, the number of rotations of the driving shaft per rotation of the belt transfer device is an integer, which prevents a misalignment between the colors due to a periodic change of the peripheral velocity of the belt resulting from an eccentric component of the driving shaft of the belt transfer device.
Similarly, the number of rotations of the photoconductive drum per rotation of the belt transfer device is an integer, which prevents a misalignment of the colors due to a periodic change of the peripheral velocity of the photoconductive drum resulting from an eccentric component of a single photoconductive drum and the gear driving the photoconductive drum.
Publication of Japanese Unexamined Patent Publication No. Hei 4-324881 uses a single photoconductive drum, and discloses employing a speed difference in a certain direction between the photoconductive drum and the transfer device to prevent sharp variations of dynamic friction due to a change in the peripheral velocity of the two.
Similarly, Publication of Japanese Unexamined Patent Publication No. Hei 8-314286 discloses rotating a single photoconductive drum faster than an intermediate transfer belt, so that a braking force overcoming the friction force on a contact portion of the two is applied on the belt driving shaft.
In order to record a full color image with high precision, an accurate alignment of four colors is necessary. When a single photoconductive drum is used, the rotational phase of the photoconductive drum usually can be synchronized with the rotation of the transfer belt and thus a relative misalignment of the color images can be prevented even though the absolute image may be expanded or contracted.
However, devices that form a color image by successively switching four photoconductive drums for all colors and superimposing these colors are subject to many problems, such as variations in the accuracy and circularity of the outer diameter of the photoconductive drums, digression from the transmission angle velocity in the parts coupling the photoconductive drum with the driving mechanism on the main body side, variations in the rotation speed of the driving mechanism itself, and variations in the positioning of the photoconductive drum into a certain position. Therefore, precise positioning is difficult and there is a need for a solution of these problems.
For example, when a color image forming apparatus uses a conventional coupling/driving mechanism with gears as shown in FIG. 15 to successively switch a plurality of photoconductive drums 218, the angular velocity of the output shaft 245 on the main body side cannot be precisely transmitted to the photoconductive drum 218, because of the differences between the different photoconductive drum gears 232. Especially in a color image forming apparatus where the image forming units integrate a photoconductive drum with other process members and the image forming units are frequently switched during use, the gear precision can worsen and adjustment for positioning can become very difficult when cheap plastic parts are used for the photoconductive drum gears 232. Therefore, the precision with which the angular velocity is transmitted from the output shaft 245 on the main body side to the photoconductive drum 218 deteriorates even further, and each photoconductive drum rotates with a different fluctuation pattern in its angular velocity. Since slippage with the transfer belt cannot cancel out variations in the recording pitch of the toner image on the photoconductive drum caused by angular velocity fluctuations, a relative misalignment of the toner image colors recorded on the photoconductive drums occurs, which results in color misalignment.
Moreover, in a color image forming apparatus that successively switches a plurality of photoconductive drums, fluctuations in the peripheral speed of the photoconductive drums resulting from an eccentricity of the photoconductive drum and flanges cannot be synchronized with the transfer belt, even when the periodic phase variations of the angular velocity of the driving system for the photoconductive drum are synchronized with the transfer belt. Therefore, the fluctuation pattern arising from different velocities in a contact portion between the photoconductive drum and the transfer belt results in different phases for each color. Moreover, fluctuations in the friction force caused by fluctuations in the distance that the photoconductive drum indents into the transfer belt result in different amplitudes and phases for each color. Therefore, it is not possible to prevent color misalignment caused by velocity fluctuations of the photoconductive drum and the transfer belt resulting from bending and twisting of the driving system under load fluctuations. Especially, when the image forming units comprising the photoconductive drums are removable, adjustments such as phase alignment become very difficult.
It is a purpose of the present invention to solve these problems of the prior art and provide a color image forming apparatus that forms a full-color image by successively switching a plurality of photoconductive drums into one image forming position and ensures very precise color alignment with a simple structure.